Texting is a feature of a phone. You cannot, without elaborate workarounds, text from a consumer computer, tablet or other device as if it was a phone. Texting requires a phone number and a phone plan.
I understand that the distinction might seem slight, but in the eyes of most US consumers, texting is distinct from a chat app that you download from an app store even if it uses your phone number.
The absolute one way that everyone with a phone has to send a textual message to another person is to text them with their phone number.
In the US, where adoption of Signal, Whatsapp, Discord, or insert hundreds of other apps is very small, the percentage of your real world contacts using a particular app is also extremely small. Convincing all of them to use Signal would certainly be great, but in reality you will be using all of those apps if you are trying to escape the interoperability nightmare that is currently texting.
Given that everyone has a phone and they are all texting already, it would be awfully nice if we could just use texting without these interoperability problems without having to manage all of the apps, and without having to remember who prefers which one.
Group texting is also hugely popular in the US. If no single third party messaging app covers the set of friends you want to group text, what do you do? You text them. Because everyone has it. Let's say when you started your group everyone was on Whatsapp. Phenomenal! Start the group on Whatsapp. Then you meet Joe, and Joe is very cool and you definitely want him in the group chat. Joe doesn't trust Meta products and doesn't want to use Whatsapp. Should Joe capitulate, install another chat app used only for a single group chat, and grant access to their device to a Meta app? Should a negotiation occur amongst the rest of the group where they select a new common app to run the group on and split the conversation history, while also adding an app that they only use for that group chat?
Let's say they choose to switch to Signal, but Josh keeps forgetting (dammit Josh) and keeps messaging the group on Whatsapp. And instead of yell at Josh that the group is on Signal now, folks reply! Because Josh's joke was super funny. Conversation also continues on Signal. Someone on Signal now does a reference to Josh's joke on Whatsapp. Joe is confused, but everyone else gets the joke. Someone realizes what happens and sends a screenshot of the joke and ensuing replies from within Whatsapp so Joe can catch up, but the messages around the joke are longer than one phone screen so there's a lot more context that he misses. Joe is annoyed but he gets over it.
A few months pass and Sandra seems to have a bug where Signal is chewing through her battery life. Since only one of her group conversations is on Signal (she uses Whatsapp mostly) and she is fine not getting the work related banter that is often the topic of the group chat. But then she finds an article that's super interesting and she wants to share it with the group. She remembers that the group moved to Signal, but who cares, that Whatsapp group still exists and there's only, like, one person that isn't in it. She sends the link in the WhatsApp group instead. This leads organically to the group wanting to get together for a holiday. They plan out that July 12th would be a perfect weekend, and since they want to do a potluck, they all choose what part of the meal they'll bring.
A few days before the potluck, someone mentions on the Signal chat that they are excited to see everyone at the potluck. Joe is very confused and asks what they mean. They realize that this was in the WhatsApp group chat and explain what everyone is bringing. Unfortunately Joe is working that weekend, and can't come.
Should the group chat reschedule?